The somatotropins (growth hormones, GH), prolactins (PRL) and placental lactogens (PL) form a family of structurally and functionally related proteins important for annual growth ad metabolism. Homologies between these proteins are also expressed at the level of interaction with cell-surface receptors, but distinct specificities exist. Human GH (hGH) has its own specific primate somatogenic receptor and will bind to both somatogenic and lactogenic receptors of animals. Little is well understood concerning the structure-function relationships of hGH interaction with receptor or its mechanism of action, due to a number of complicating factors (e.g. bioactive contaminants, difficult bioassays, multiple forms of the hormone, receptor population heterogeneity). Previous studies of insulin have indicated that iodination of a protein can alter its activity and/or affinity for receptor. Since much work investigating hGH action uses 125I-preparations of hGH, it is proposed to systematically isolate, identify and characterize the 125I-iodoisomers of hGH generated by standard iodination procedures. Techniques such as gel filtration and high performance liquid chromatography (HPLC), polyacrylamide gel electrophoresis, isoelectric focusing, and paper chromatography and electrophoresis, and tryptic peptide mapping will be employed to characterize the starting hGH and subsequent iodoisomers. The purified tracers will be further characterized for binding to human lymphocytes, rabbit mammary gland membrane and rabbit liver membrane somatogenic and lactogenic receptors as well as in radoimmunoassays, and in vitro lactogenic and somatogenic bioassays using prepared 127I-iodoisomers. HGH is active in nonprimates, but nonprimate GH is not active in man, thus there has been a shortage of hGH available for the clinical treatment of GH-dependent growth disorders. Now, however, biosynthetic methionyl-hGH (mhGH) synthesized in E. coli using recombinant DNA technology is undergoing testing to allow its use clinically. Studies of mhGH will be conducted in parallel with those of highly purified hGH and the results should contribute significantly to our understanding of somatotropin: receptor interactions, as well as hasten the evaluation of the critically needed mhGH.